Superconducting quantum interference devices (SQUIDs) are extremely sensitive detectors of magnetic flux. When paired with appropriate feedback and readout electronics, SQUIDs can detect magnetic fields corresponding to fractions of a flux quantum (.PHI..sub.0).
Because of the nature of magnetic fields, SQUIDs can be used in applications where light does not penetrate and sound is distorted. SQUIDs can be used to detect underwater objects such as mines and submarines, or to determine probable locations of oil and mineral deposits. They can detect magnetic signals produced by the body as well, detecting the firing of neurons in the is brain in magnetoencephalography (MEG), or disease in soft tissues in magnetic resonance imaging (MRI).
In certain cases, however, this extreme sensitivity to magnetic fields can be detrimental to a SQUID's performance in an application. For example, a large background field can mask a smaller field that is of interest. In this case the SQUID must be very sensitive to see the small target signal. However, if the large background field penetrates even a small portion of the SQUID's field of view it can swamp the signal.
Typically in such uses of SQUIDs as magnetometers, both the SQUID and the object under study are located within a magnetic shield. This can be a "shielded room" which is available commercially and which is simply a room built to reject any external magnetic or s electromagnetic signals. Another option is to completely enclose the object and sensor in a superconducting enclosure. Since superconductors are perfect diamagnets, no magnetic field can penetrate a superconducting plate or box. (Under certain conditions, magnetic flux can penetrate a superconductor. However, it is easy to predict the magnetic field strength which will be shielded by any superconducting shield, and to design the shield to accomplish this task.) Unfortunately, this shielding is not possible in all situations.
One example of such a situation is the use of SQUIDs for magnetic microscopy and non-destructive testing (NDT), or evaluation (NDE). In these applications, spatial resolution is very important. The change in a magnetic signature over regions a few micrometers in diameter can be important for pathologists looking at a biopsy sample or for aircraft maintenance engineers looking for an incipient crack in a corroded weld. The small field of view and the typically small changes in magnetic field that must be detected require the use of extremely sensitive SQUIDs. At the same time, the small field of view or the fineness of the array precludes the use of external shields to block background magnetic fields from equipment like computers or from the earth itself. The combination of very sensitive detectors and an unshielded environment places stringent requirements on the magnetic sensing system.